This invention relates to a control method of oxygen density in a combustion control system, and more particularly to a method for controlling oxygen density in exhaust gas, that is created by burning fuel, in accordance with a set value which is varied by variation in the combustion load.
In an ordinary process control apparatus for a combustion furnace, fuel supplied to the furnace is burnt at a predetermined fuel-air mixing ratio, and the density of oxygen in the exhaust gas is detected by a density sensor provided in an exhaust port of the combustion furnace, while a temperature sensor is provided to detect the temperature of the furnace.
In this case, an appropriate control of the fuel-air mixing ratio is important for the combustion system in order to effect a high efficiency combustion, and when the combustion efficiency becomes low, harmful gases such as CO, NO.sub.x and dense smoke are exhausted, which gives rise to a pollution problem.
In order to ensure an optimum control of the fuel-air mixing ratio in the conventional process control apparatus, the output of the oxygen density sensor is applied to an oxygen density controller which also receives a set value delivered from an oxygen density setting device. The oxygen density controller compares the detected value of the oxygen density with the set value and delivers an output error signal which is thereafter utilized for controlling the combustion system. The oxygen density setting device presents a set value that is varied in accordance with the variation in the combustion load. On the other hand, an electric signal delivered from the temperature sensor is applied to a temperature controller provided in a combustion control apparatus.
Ordinarily, when fuel is burned in a burner of a combustion furnace, the fuel is mixed with air at a predetermined ratio. The fuel-air mixing ratio which is more accurately defined by the flow rates in weight of fuel and air is ordinarily termed an air excess ratio .mu.. Ordinarily, the air excess ratio .mu. is maintained at a constant value .mu..sub.o substantially equal to 1.1.
Thus when an error is found in the above described process control apparatus between the set value and the actually detected value of the oxygen density, an oxygen density controller receiving the set value and the detected value delivers an output signal adapted to reduce the difference between the two values to zero. The output signal of the controller is converted into a correcting value .DELTA..mu., and applied to the combustion control apparatus. In the combustion control apparatus, the air excess ratio .mu. is corrected in accordance with EQU .mu.=.mu..sub.o =.DELTA..mu. (1)
and a control operation is carried out so that the actually detected oxygen density in the exhaust gas is made equal to the set value.
The above described equation (1) can be rewritten as follows. EQU .mu.=.mu..sub.o +k(MV-x.sub.o)/100 (2)
wherein
k is a constant, PA1 x.sub.o is a constant which is ordinarily selected to 50, and PA1 MV represents the output of the oxygen density controller which is ordinarily varied between 0 and 100.
When the correcting value .DELTA..mu. is calculated from the output MV of the oxygen density controller as indicated in the equation (2), the combustion control apparatus controls the mixing ratio of fuel and air in accordance with the excess ratio .mu. defined by equation (2) and also with the output signal from the temperature controller which is provided in the apparatus for controlling the temperature in the combustion furnace.
The control method of the oxygen density in the exhaust gas is required to exhibit the following features.
(1) The oxygen density set value is not constant, but is varied in accordance with combustion load that is expressed by the flow rate of the fuel.
(2) Since the control process includes dead time of a substantial length, a sampling PI (proportional and integral) control is utilized. In this case, for the calculation of the correcting value .DELTA..mu. of the air excess ratio .mu., the control frequency must be so selected that the time interval between the successive controls is made longer than the dead time, and the output of the regulator is renewed frequently.
The above described conventional control method is found to be disadvantageous in that a control error which occurs during a leisure time of the control cannot be reflected to the control of the oxygen density. Furthermore, in the conventional control method, the output of a calculator which calculates the equation (2) is a fixed value. However, in practical plants, the combustion condition varies depending on the set value of the oxygen density, and therefore the process gain of the control must be thereby varied. The conventional method lacks such a versatility of control.